The related art of the present invention is described here by taking the hydraulic excavator shown in FIG. 5 as an example.
This hydraulic excavator is provided with a lower travel body 1 of a crawler type, an upper swivel body 2 mounted swivelably about a vertical axis X with respect to the ground surface on, the lower travel body 1, and an attachment 9 installed on the upper swivel body 2; the attachment 9 includes a boom 3, an arm 4, a bucket 5 and respective hydraulic actuators for actuating the boom 3, arm 4 and bucket 5, namely, a boom cylinder 6, an arm cylinder 7 and a bucket cylinder 8. The hydraulic excavator further includes, as other hydraulic actuators, left/right travel motors which cause the lower travel body 1 to travel by driving respective left and right crawlers included in the lower travel body 1, and a swivel motor which drives and swivels the upper swivel body 2.
In a hydraulic excavator of this kind, a three-circuit/three-pump system such as that shown in Patent Document 1 is known as a drive system for ensuring independence between the swivel operation and the operations of the other actuators. In this system, the hydraulic circuits for this driving are divided into: i) a first circuit which includes one travel motor of the left/right travel motors and the boom cylinder 6; ii) a second circuit which includes the other travel motor and the arm cylinder 7; and iii) a third circuit which includes the swivel motor. The first to third circuits are provided with first to third pumps respectively.
The circuit disclosed in Patent Document 1 further includes a merge valve which switches the path of the hydraulic fluid discharged by the third pump. This merge valve, which has a first position as a neutral position and a second position, is switched from the first position to the second position in the case of combined operations in which a boom raising operation and a swivel operation are carried out simultaneously. In the second position, the merge valve forms a fluid path for supplying third pump fluid, which is the hydraulic fluid discharged by the third pump, to the boom cylinder in parallel with the swivel motor, that is, for causing the third pump fluid to merge into the first pump fluid, which is the hydraulic fluid discharged by the first pump.
The merge valve, however, has a response delay when switching from the first position to the second position, which generates a risk of causing a shock in the swivel action. For example, in a case of starting a boom raising operation while a swivel operation is applied, if the merge valve is switched from the first position to the second position simultaneously with the start of the boom raising operation, then the maximum pressure (swivel pressure) of the swivel motor declines gradually during the boom raising operation; however, if the merge valve is switched to the second position after a delay from the start of the boom raising operation (in other words, if the merge valve is switched to the second position in a state where the boom raising operation has progressed to a certain extent), then the third pump fluid is switched suddenly from a state of being supplied only to the swivel motor, to a state of being supplied in parallel to the swivel motor and the boom cylinder, which causes the maximum pressure (swivel pressure) of the swivel motor to change suddenly from a relief pressure to a boom operating pressure, thereby generating possibility of giving marked shocks to the swivel operation. Shocks of this kind can be a cause of decline in the operation performance.
Patent Document 1: Japanese Patent No. 3681833